1. Field of the Invention
The present invention relates to moisture-curable compounds containing aromatically-bound isocyanate groups and alkoxysilane groups incorporated through aspartate groups, which can be cured in the presence of moisture to form coatings, adhesives and sealants.
2. Description of the Prior Art
It is known that polyisocyanate resins are curable in the presence of atmospheric moisture to form polyurea coatings. During the curing mechanism an isocyanate group reacts with moisture to form an amino group, which then reacts with another isocyanate group to form a urea group. Even though the polyisocyanate resins possess good adhesion to substrates, there are applications where an improvement in adhesion is needed. In addition, even though moisture-curable resins based on aromatic polyisocyanates have reasonably fast dry times, there is always a need to improve the cure rate of these resins.
It has been suggested in U.S. Pat. Nos. 3,420,800 and 3,567,692 that the curing rate of moisture-curable polyisocyanates can be increased by incorporating either aldimines or ketimines. It is stated that the reaction of moisture with an aldimine or ketimine to form the corresponding amine is faster than the reaction of moisture with an isocyanate group to form an amine. A disadvantage of the use of aldimines and ketimines to accelerate the cure of polyisocyanates is that it requires the preparation of an additional component and requires some type of metering equipment to ensure that the two components are blended in the proper proportions.
It is an object of the present invention to provide compositions having good adhesion to substrates and which cure rapidly to form high quality coatings.
This object may be achieved with the polyisocyanates according to the present invention, which have been modified to contain alkoxysilane silane groups incorporated through aspartate groups. The fact that it is possible to maintain or even increase the cure rates in accordance with the present invention is surprising because alkoxysilane groups, which are also curable in the presence of moisture, cure more slowly than polyisocyanates. However, when both isocyanate groups and alkoxysilane groups are present, the combination of curing rate and surface appearance is improved.
Copending application, U.S. Ser. No. 09/058,072, discloses water dispersible compounds containing isocyanate and alkoxysilane groups. The water dispersible compounds must be prepared from a polyisocyanate component having a minimum average functionality of 2.4 and be rendered water dispersible by the use of either ionic or nonionic hydrophilic groups.
Copending application, U.S. Ser. No. 08/992,163, discloses the preparation of polyurethane-urea dispersions containing alkoxysilane groups incorporated through aspartate groups. However, because the resulting polyurethane dispersions are dispersed in water, they do not contain any unreacted isocyanate groups.